Analog instrumentation remains the most widely used and preferred method of displaying automobile data to the driver. This is due to their simple function and ability to be adapted to many different styles. Even expensive automobile models that essentially have an unlimited instrumentation budget commonly choose analog gauges. Future requirements for instruments will unquestionably include analog gauges. A need is seen for new technology to help control the rising costs of instrument clusters by enabling a simpler more flexible instrument cluster design concept.
Most current production automotive analog gauges utilize either air core, D'arsonval, or bi-metal gauge technology.
One main problem with current air core gauges is their size and bulk. Mechanical complexity in the instrument housing, face plate and connections are required to mount and constrain the gauge mechanisms. The instrument housing has evolved into a very complex part that requires tooling with long lead times and high cost. This high tooling cost is further magnified when "late changes" are required after the design is released for production. Automation of the assembly process requires specially designed equipment for each type of instrument cluster produced. The continual capital investment in this manufacturing equipment each time a new model is introduced drives the instrumentation costs even higher.
The new micro-miniature gauge of the present invention does not require the complex mounting methods and housing complexity that today's larger gauges need. The new gauge is directly mounted to the printed circuit board which becomes the support structure for the entire instrument cluster.
The new micro-miniature gauge becomes an enabling technology for flexible instrument cluster designs, miniature telltale modules, and analog projection HUDs (heads up display).
Air-Core or Cross-Coil gauges have been used in automotive instruments since they were first invented in 1965 (see U.S. Pat. No. 3,168,689). As shown in FIG. 1, a prior art crossed coil gauge 10 as shown utilizes a two pole radially charged cylindrical magnet 12 attached to a coaxial shaft 14. The magnet rotates with the shaft in response to a resultant magnetic flux vector. This flux vector is generated by two coils 16 wound one over the other encircling the magnet. The coils are "air-core", i.e., they contain no iron, and the first coil axis is perpendicular to the second coil. The coils are surrounded by an iron ring or an iron can 18 to provide shielding from other external magnetic fields. The magnet is housed in a plastic bobbin 20 that serves as bearing, damping fluid container, coil bobbin, iron can holder and gauge mounting means. Silicone damping fluid 22 fills the fluid container cavity formed in the bobbin and restrains magnet rotation. Gauge 10 has a flat upper mounting surface 24 and a series of electrical terminals 26 on the opposite surface.
Over the years several refinements have been proposed for aircore gauges. Most recently, U.S. Pat. No. 4,760,333 shows a novel way to provide a magnetic shield utilizing a wound strip of amorphous metal alloy. This replaces the conventional iron can or ring to provide magnetic shielding. U.S. Pat. No. 4,827,210 shows a modification to the plastic bobbin to facilitate equal coil winding lengths to produce a more linear gauge. U.S. Pat. No. 4,992,726 shows woven (interlaced) coil windings to produce a more linear gauge. U.S. Pat. No. 5,017,862 shows improved bearing design and bobbin structure. U.S. Pat. No. 5,038,099 teaches a radial air-core gauge design that balances the coil windings and produces a thinner gauge. U.S. Pat. No. 5,095,266 shows a method of containing viscous damping fluid for more uniform damping of the gauge.
Even though many incremental improvements have been shown in the literature, several problems remain with current production air-core gauges. The first problem is size. Most air core gauges are from 3/4 to 11/4 inches in diameter and 1/2 to 1 inch long. They typically require complex mounting methods involving various screws, clips and plastic molded parts. Special machines and processes are often designed to assemble the gauges into instrument clusters. The gauge thickness can control the minimum thickness of the instrument cluster and a larger gauge diameter makes backlighting the instrument dials difficult. Complex plastic light pipes are often used to light gauge dials and pointers to bring light from behind and around to the front of the gauge. Automated gauge assembly directly to electronic circuit board assemblies is desirable but difficult with current large air-core gauge designs.
A second unresolved problem involves the use of a liquid damping fluid in the air-core gauge. This is most commonly a silicon fluid of the appropriate viscosity. A messy production process is required to deposit the fluid inside the gauge bobbin assembly. Migration of the fluid often occurs while the gauge is being transferred or stored or in actual use, resulting in lesser or sometimes no damping of the gauge after a period of usage. Various attempts have been made to eliminate problems with viscous fluid damping. U.S. Pat. No. 5,095,266 shows a recent attempt.
Magnetic eddy current damping has been used in devices other than air-core gauges to eliminate a viscous fluid. One of the most common devices utilizing eddy current damping is a watt-hour meter used by electrical utilities. The use of eddy current damping in these devices is described in U.S. Pat. Nos. 4,238,729 and 4,238,730. The eddy current damping provides a continuous drag torque on a rotating electrically conductive disk or cylinder which balances the disk drive torque providing a disk speed proportional to the rate of power consumption. It is generally not used as a transient torque vibration damper in this application. U.S. Pat. No. 3,786,685 shows a copper cup-shaped ring that rotates in a magnetic field to dampen transient motion in a gyro. U.S. Pat. No. 3,983,478 discloses a moving coil instrument with copper rings for eddy current damping. These rings are welded to the moving coil and cut through a magnetic field to slow their rate of rotation. These eddy current damping devices all depend on high magnetic flux densities generated by using small air gaps and flux concentrating iron pole pieces or multiple magnets. They are generally bulky and heavy because of the high flux requirements for damping.
It has been observed that all current production air core gauges utilize the iron can or ring to provide magnetic shielding. Because of the design geometry and the magnets used, the can has little effect on the flux linking the gauge coils. Thus, the iron can or ring has little effect on the gauge torque which is primarily limited by the magnet materials, the gauge geometry and the number of amp-turns produced by the coil windings.
Air-core gauges have been used in various "telltale" designs to rotate icon masks in front of a light to indicate a failure in one of a number of automotive systems (see U.S. Pat. No. 3,660,814). In these applications a problem exists that no detente action is provided in the gauge to "hold" a position. Therefore, continuous power is generally required. A need exists to provide a method of modifying the magnetic reluctance of the air core gauge to provide unique torque characteristics in the gauge for detente action or to linearize the gauge.
In summary, several current air-core gauge problems have been identified. These problems are: a.) large gauge size; b.) unreliable liquid damping; c.) inefficient magnetic flux utilization; and d.) constant magnetic reluctance.